This invention relates to an austenitic stainless steel which is substantially fully non-magnetic in the hot worked and forged condition, to oil well drill collars fabricated therefrom and to a method of making drill collars which have high strength and improved resistance against stress corrosion cracking.
Drill collars are used in oil well drilling in order to provide the proper loads on the drill bit. In directional drilling it is necessary to provide electronic measuring and guidance instruments in the drill stem. Non-magnetic drill collars are therefore necessary in order to ensure proper functioning of the electronic instruments. Due to the greater drilling depths presently being explored, higher temperatures are encountered together with chloride and sulfide containing liquids. Such conditions exacerbate stress corrosion cracking problems in austenitic steels.
The non-magnetic steel of the present invention exhibits a surprising increase in strength in the hot worked condition and improvement in stress corrosion cracking resistance in comparison to a conventional prior art steel widely used for fabrication of drill collars. Moreover, the steel of the invention can be fabricated into drill collars without the "warm working" treatment required for the conventional steel, and without cold working.
U.S. Pat. No. 3,082,083, to Levy and Goller, discloses a prior art steel which has been widely used in the fabrication of drill collars and describes the forging and warm working treatments to which the steel is subjected, viz., rough forging at 1800.degree.-2100.degree. F. (982.degree.-1149.degree. C.), reheating and further forging at 1300.degree.-1500.degree. F. (704.degree.-815.degree. C.) with a reduction of about 20%. When forged at 1800.degree.-2100.degree. F. the yield strength is about 60 ksi. Reheating and warm working increases the yield strength to about 100 ksi. Cold working the steel of this patent raises the yield strength to above 100 ksi. The steel of this patent consists essentially of 0.10% to 0.25% carbon, 7% to 14% manganese, 12% to 18% chromium, greater than 5% to 15% nickel, 0.15% to 0.5% nitrogen, and balance essentially iron. In the cold or warm worked condition the magnetic permeability is alleged to be not greater than 1.007.
U.S. Pat. No. 3,940,266 and a division thereof, U.S. Pat. No. 3,989,474, to Goller and Espy, disclose an austenitic stainless steel combining good stress corrosion cracking resistance and cryogenic toughness, consisting essentially of 0.06% to 0.12% carbon, 11% to 14% manganese, 15.5% to 20% chromium, 1.1% to 2.5% nickel, 0.20% to 0.38% nitrogen, 0.5% maximum copper, 0.5% maximum molybdenum, and balance essentially iron. Another embodiment contains 0.01% to 0.06% carbon and 2.5% to 3.75% nickel, with the ranges of all other elements remaining the same.
An austenitic steel sold by Armco Inc. under the trademark "Aquamet 18" for boat shafts, contains 0.15% maximum carbon, 11.0% to 14.0% manganese, 16.5% to 19.0% chromium, 0.5% to 2.50% nickel, 0.20% to 0.45% nitrogen, and balance essentially iron.
U.S. Pat. No. 3,112,195 discloses an austenitic steel alleged to be suitable, in the cold worked condition, for drill stems (collars) in oil well drilling. In broad ranges the steel of this patent contains up to 0.35% carbon, 12% to 25% manganese, 10% to 20% chromium, up to 5% nickel, 0.05% to 0.50% nitrogen, up to 1% molybdenum, and balance essentially iron. Optional alloying additions include tungsten, titanium, columbium (and/or tantalum), boron, vanadium, copper and cobalt, in a total amount not exceeding 10%.
U.S. Pat. No. 3,904,401, to Mertz et al, discloses an austenitic steel consisting essentially of 0.25% maximum carbon, 15% to 20% manganese, 16% to 22% chromium, 3% maximum nickel, 0.2% to 0.8% nitrogen, 0.5% to 3% molybdenum, 0.5% to 2% copper, 0.5% maximum sulfur, and balance essentially iron.
Despite the availability of the alloys disclosed in the above-mentioned patents, and other alloys such as K-Monel (containing at least 63% nickel and at least 25% copper), nominal 18% chromium-8% nickel steels and 15% chromium-5% nickel steels, for fabrication into oil well drill collars, there is still a need for an alloy which combines high stress corrosion cracking resistance, low alloy cost, and high strength in the hot worked condition and low magnetic permeability. Thus, the steel of U.S. Pat. No. 3,082,083 is deficient in stress corrosion cracking resistance and requires warm working to achieve the desired strength levels. The steels of U.S. Pat. Nos. 3,112,195; 3,940,266 and 3,989,474 require cold working to achieve the desired strength levels. Alloys such as K-Monel and 18-8 and 15-5 stainless steels are prohibitive in cost because of the high nickel contents thereof.
It has now been discovered, in accordance with the present invention, that observance of critically narrow percentage ranges of the essential elements carbon, manganese, chromium, nickel, nitrogen and copper, and critical proportioning therebetween, particularly carbon and nitrogen, result in a non-magnetic, substantially fully austenitic stainless steel which exhibits improved stress corrosion cracking resistance, adequate toughness, good ductility, and a high yield strength in the hot worked condition which makes it unnecessary to resort to the prior art warm working practice of U.S. Pat. No. 3,082,083 and the cold working described in U.S. Pat. No. 3,112,195. When fabricated into drill collars by forging and hot working, the magnetic permeability of the steel of the invention does not exceed 1.004 even if cold worked.
The present invention thus provides an austenitic stainless steel having a 0.2% yield strength of at least 85 ksi, a stress corrosion resistance of greater than 1,000 hours under stress of 25 ksi in boiling 42% magnesium chloride solution, and a reduction of area of at least about 50% in the hot worked condition, and a magnetic permeability not greater than 1.004 at 500 oersteds in the cold worked condition, said steel consisting essentially of, in weight percent, from 0.12% to about 0.20% carbon, 11% to about 14% manganese, about 0.80% maximum silicon, about 0.04% maximum phosphorus, about 0.025% maximum sulfur, about 16% to about 19% chromium, about 1.5% to 2.7% nickel, 0.30% to 0.45% nitrogen, 0.5% to about 1.0% copper, about 0.75% maximum molybdenum, and balance essentially iron, with the carbon:nitrogen ratio being not greater than about 0.6:1.
The term "balance essentially iron" is intended to include iron with minor amounts of unavoidable impurities which do not adversely affect the properties of the steel.
The invention further provides a non-magnetic oil well drill collar produced by hot forging having a 0.2% yield strength greater than 85 ksi at the longitudinal outside diameter position, a stress corrosion cracking resistance of greater than 1000 hours under stress of 75 ksi in boiling 5% NaCl+0.5% acetic acid solution, and a magnetic permeability not greater than 1.004 at 500 oersteds, said collar being hot forged from an austenitic stainless steel consisting essentially of, in weight percent, from 0.12% to about 0.20 carbon, 11% to about 14% manganese, about 0.80% maximum silicon, about 0.04% maximum phosphorus, about 0.025% maximum sulfur, about 6% to about 19% chromium, about 1.5% to 2.7% nickel, 0.30% to 0.45% nitrogen, 0.5% to about 1.0% copper, about 0.75% maximum molybdenum, and balance essentially iron, with the carbon:nitrogen ratio being not greater than about 0.6:1.
In accordance with the invention, a method of fabricating a non-magnetic oil well drill collar, having a 0.2% yield strength greater than 85 ksi at the longitudinal outside diameter position, a stress corrosion cracking resistance of greater than 1000 hours under stress of 75 ksi in boiling 5% NaCl+0.5% acetic acid solution, and a magnetic permeability not greater than 1.004 at 500 oersteds, comprises the steps of providing a steel billet consisting essentially of, in weight percent, from 0.12% to about 0.20% carbon, 11% to about 14% manganese, about 0.80% maximum silicon, about 0.04% maximum phosphorus, about 0.025% maximum sulfur, about 16% to about 19% chromium, about 1.5% to 2.7% nickel, 0.30% to 0.45% nitrogen, 0.5% to about 1.0% copper, about 0.75% maximum molybdenum, and balance essentially iron, with the carbon:nitrogen ratio being not greater than about 0.6:1, heating said billet within the range of about 982 .degree. to about 1149.degree. C. (1800.degree. to 2100.degree. F.), and hot forging the billet to final diameter without intermediate reheating at a finishing temperature of at least about 677.degree. C.